As is well known, polyblends of rubber with monoalkenyl aromatic polymers have significant advantages in providing compositions of desirable resistance to impact for many applications. Various processes have been suggested or utilized for the manufacture of such polyblends including emulsion, suspension and mass polymerization techniques, and combinations thereof. Although graft blends of a monoalkenyl aromatic monomer and rubber prepared in mass exhibit desirable properties, this technique has a practical limitation upon the maximum degree of conversion of monomers to polymer to polymer which can be effected because of the high viscosities and accompanying power and equipment requirements, which are encountered when the reactions are carried beyond a fairly low degree of conversion after phase inversion takes place. As a result, techniques have been adopted wherein the initial polymerization is carried out in mass to a point of conversion beyond phase inversion at which the viscosity levels are still of practical magnitudes, after which the resulting prepolymerization syrup is suspended in water or other inert liquid and mass polymerization of the monomers carried to substantial completion.
Stein, et.al. in U.S. Pat. No. 2,862,906 discloses a mass suspension method of polymerization styrene having diene rubbers dissolved therein with the rubber being grafted, inverted and dispersed as rubber particles under agitation. After phase inversion, the viscous mixture is suspended in water and mass polymerization is completed producing a polyblend in the form of beads.
Such mass suspension processes are used commercially, however, present the economic problems of batch operations requiring long cycles at relatively low temperatures to control the heat of polymerization. Continuous mass polymerization processes have great economic advantages if they can be run at higher temperatures and higher rates with the necessary control of the great heats of polymerization. In the case of polyblends, the dispersed rubber phase must be formed and stabilized as to its morphology, bringing it through the continuous polymerization of the rigid matrix polymer phase so that the physical properties of the polyblend meet exacting property specifications.
Various methods have been developed for the continuous mass polymerization of polyblends. Ruffing, et.al., in U.S. Pat. No. 3,243,481 disclose a process wherein diene rubbers are dissolved in predominantly monovinylidene aromatic monomers and polymerized in four reaction zones.
U.S. Pat. No. 3,903,202 discloses a process of the continuous mass polymerization of polyblends using two reactors as a more simple process for mass polymerizing such polyblends.
Hence, the mass polymerization of rubber-monomer solutions by batch or continuous mass polymerization are known as well as batch mass-suspension processes in that the suspended droplets polymerize by mass polymerization kinetics and the beads formed are minature mass polymerization systems. The present process then is adaptable to mass polymerization processes of the type described.
The above processes all produce polyblends that have a dispersed and grafted rubber phase. It has been found that the polyblends are toughened by the rubber phase in direct proportion to the rubber content. Beyond the rubber content it has been found that the efficiency of the rubber in toughening is greatly enhanced by grafting the rubber with the polymer of the matrix phase to provide an interfacial compatibility between the rubber phase and the matrix phase.
Generally, the rubber is grafted from about 10 to 100 percent with the matrix monomers with the rubber as a substrate and the graft monomers forming graft polymers as superstrate.
Prior art processes have used free radical catalysts to promote polymerization of the monomers and also extract the allylic hydrogen from the rubber so that the monomers would graft to the rubber more efficiently.
It has been found that higher levels of graft are formed by using higher levels of catalyst, however, the increased use of catalyst lowers the molecular weight of the matrix phase giving a net lowering of impact strength. Hence, a need exists for a process that will increase the grafting of the rubber phase yet allows the matrix phase polymers to reach optimum molecular weights consistent with optimum physical properties.
It is the objective of the present invention to provide an improved mass polymerization process for rubber-monomer solutions that will provide maximized grafting of the rubber phase in conjunction with an optimized matrix phase molecular weight giving polyblends with improved physical properties.